This is a streamlined derby car I did for middle daughter. The hole in the top middle was so we cold add pennies to dial in the weight on the official track scale.There are 2 versions the regular has all 4 tires at an equal height, the "lifted tire" version has one of the front tires .100" higher than the others. The theory there is less rotational inertia, will allow the car to go faster. There are 3 holes that run about 2/3 the length in the back, I added 1/4-20 socket head cap screws in there to get my mass up and have my center of mass closer to the back wheels. I printed is only about 15-20% infill. This made it way under our weight limit and then I added the weight where I wanted it. I added washers into the figure 8 cutout in the bottom to keep the CG low.